Conventionally, as small antennas that are used in microwave or millimeter-wave radar or communication devices, a waveguide slot antenna in which a waveguide is formed with a metallic material and in which air in the waveguide is used as a medium to transmit high frequency signals, or a triplate antenna that is constituted by a resin substrate and a metallic plate, and in which air between the substrate and the metallic plate is used as a medium to transmit high frequency signals are known.
FIG. 6 is a cross section illustrating an example of a structure of radar equipped with a triplate antenna. As shown in FIG. 6, a radar apparatus 201 has such a structure that a triplate antenna 3 and a circuit substrate 2 are fixed by fixing screws 14 to a waveguide plate 10 that is sandwiched therebetween.
The triplate antenna 3 has such a structure that two pieces of metallic plates 7 are opposed to each other at a predetermined interval, and that a resin antenna substrate 4 is layered on one of the metallic plates 7. On a surface of the antenna substrate 4, multiple antenna devices 5 and an antenna line 6 that propagates high frequency signals to the antenna devices 5 are arranged. Moreover, in the metallic plate 7 on which the antenna substrate 4 is not arranged, a waveguide converter 8 is formed at a position opposing to a hollow waveguide 11, on a surface opposing to the hollow waveguide 11, and vias 9 are opened at positions opposing to the antenna devices 5, respectively.
In the circuit substrate 2, the hollow waveguide 11 is arranged piercing therethrough, and a predetermined conductive pattern 13 is formed on both principal surfaces of the circuit substrate 2. Furthermore, the internal wall of the hollow waveguide 11 is covered with the conductive pattern 13. At a position opposing to one opening of the hollow waveguide 11, a high frequency module 1 is arranged. The hollow waveguide 11 extends to the antenna substrate 4 piercing through the waveguide plate 10. In addition, choke slots 12 are formed so as to surround the hollow waveguide 11.
In the radar 201 having such a configuration, the high frequency module 1 and the antenna substrate 4 forms a structure of a waveguide converter with which high frequency signals can be propagated in both ways as indicated by a dashed arrow A shown in the figure, the high frequency module 1 and the antenna substrate 4 are connected through the hollow waveguide 11, and further, the choke slots 12 are formed so as to surround the hollow waveguide 11. Therefore, the transmission loss between the high frequency module 1 and the antenna substrate 4 can be reduced.
In addition, the structure of an antenna using two pieces of the metallic plates 7 opposing to each other has another advantage that the waveguide converter 8 provides for matching to suppress degradation of the transmission characteristic (loss and reflection) that occurs at a connecting point at which waveguides having different shapes are connected (connecting point between the metallic plate 7 and the antenna substrate 4), a point at which waveguides are branched or combined, or the like can be formed by providing a slot or a projection in the metallic plate 7 (for example, refer to Patent Literature 1).